This invention relates to a method and circuit for determining the capacity of a battery and its cells and is particularly adapted to determining the capacity in a minimum amount of time. The time can be further reduced by the state-of-charge reading during charge.
In many applications it is important to know the maximum capacity of a battery as well as the cell capacities because it can, in some instances, be disastrous if the battery fails in operation. One such use of batteries is in the emergency powering of equipment, such as communications systems. Another such use is in the starting and emergency power for aircraft. Typically, vented nickel-cadmium (Ni-Cad) batteries are used for this latter purpose. Such batteries have a long life and may be recharged many times. However, over a period of time one or more cells of the battery may fail or the overall condition may degrade to the point that the maximum or full-charge capacity of the battery is too low or unreliable to be used. The maximum capacity of such batteries is periodically checked to determine if the battery is satisfactory to remain in use or should be replaced.
The known methods and apparatus for analyzing batteries to determine the maximum capacity involve excessively long periods of time. One common approach in analyzing Ni-Cad batteries that are used in aircraft is to first charge the battery and then fully discharge the battery down to a generally accepted full discharge voltage of typically 1 volt average per cell to determine the ampere-hour capacity of the battery to see if this maximum capacity is sufficiently high for the battery to be re-used. This discharge test usually takes about one or two hours. Charging techniques for the charge cycle require anywhere from one hour to four hours or more to charge the battery. Consequently, this process takes at least two hours to determine the capacity of a battery and then at least another hour to recharge the battery if the capacity of the battery is found to be acceptable.